It is often of interest to be able to determine the type of biological tissue that was surrounding a probe inserted into the human or animal body at locations or under circumstances that do not allow for inspection by other means. When administering drugs, tracers or taking biopsies, it is often critical to position the tip of a needle at a specific position or in a specific tissue type. Serious implications and undesired results may incur if the needle unintentionally hits or penetrates veins, arteries, lungs or nerves. In regional anaesthesia, such as peripheral nerve blocks, drugs must be injected close to a nerve. Wrong or imprecise injection of the anaesthesia results in little or no effect. Injections within the nerve, on the other hand, can give nerve damage that can lead to persistent motor or sensory impairment and debilitating neuropathic pain. Biopsies are carried out by insertion of a needle that can cut and extract a small tissue sample. It may, however, be difficult to ensure that the extracted sample is of the desired tissue type without some form of guidance.
To avoid potential complications, some sort of guidance is used in critical cases. High risk procedures are often carried out under guidance of advanced apparatuses such as X-ray, CT, ultrasound, or MR imaging. Ultrasound images reveal abrupt changes in acoustical impedance, but are insensitive to homogeneous regions. X-ray images have poor contrast in soft tissues. MRI is sensitive to soft tissue properties, but is complicated to use and requires MRI compatible needles and moving of the patient to a MRI clinic. It is a common disadvantage of the use of these types of guiding techniques that the required equipment is complicated and expensive in relation to the frequent and relatively simple task of inserting a needle with reasonable precision.
US 2003/109871 describes an apparatus for detecting and treating tumours using localized impedance measurement. The impedance measurement configuration is described in paragraph [0060] in relation to FIG. 3A. From here it appears that members 22m define sample volumes by means of conductive pathways (22cp) to either between each other or to a common ground electrode (22g or 22gp). It thus appears that the apparatus always measure the impedance of a sample volume of interest (5sv). By switching the electrodes between which the measurement is made, the conductive pathway 22cp is changed which again alters the shape and size of the associated sample volume. It is a disadvantage of the apparatus described in US 2003/109871 that it measures impedance in relatively large volumes.
WO 2007/075091, which describes earlier work forming a starting point for the work of the current inventors, describes the use of a probe such as a needle to determine data that relates to tissue type. As described in this document, a probe can be used to obtain an electrical impedance spectrum relating to the tissue adjacent a specified location on the probe, typically a tip of a needle. It would be highly beneficial for the user to be able to determine the type of tissue from the impedance spectrum. This would mean that the tissue type can be determined without the need for further invasive or non-invasive procedures. However, WO 2007/075091 provides no teaching regarding general techniques for determining specific tissue types, and in fact only describes the possibility of a comparison with earlier impedance spectra with newly measured impedance spectra, thereby making a suggestion of a tissue type match when the newly measured spectra is similar to an earlier measured spectra with known tissue type. This process is limited in its accuracy and utility. The impedance spectra can vary not only with tissue type, but also with physiological conditions. The impedance spectra from different patients or using different measurement set-ups will vary. Thus an exact match may not be possible and even if achieved it may not be accurate. What is needed therefore is a more accurate and more generally applicable way to use the impedance spectra to determine tissue type.